Can I get an IK constraint w/ stretch to only scale bones on their Y axes?
IK stretch bones do stretch in all 3 axes and can't behave like you want.
The solution and common method is to


duplicate all the bones in the chain (except the control) and un-parent them from each other
for each bone_copy set Copy Location and Copy Rotation constraints targeting the original bone
for each bone_copy set Stretch To constraint targeting the next bone in the chain. This constraint should have Volume set to XZ to maintain reality or to None if you only want to stretch in Y axis
bind the mesh to the copied bones